In working on some of my family history scans last week, I had some very large TIF files that had been scanned in grayscale. Opening them in showFoto and digiKam showed large, pixelated patches of black. At first I thought it was a scanning error, but I quickly found that the patches weren’t in the image- they seemed to just be super-imposed on them. In showFoto (which by the way is a fantastic photo editor and toolbox), go to ‘Settings’ > ‘Configure showFoto’ and in the window that pops up, the first tab that’s selected will be ‘Image Editor’. A little way down in the window is section called ‘Exposure Indicators’. To eliminate the black patches on my scans, I adjusted the ‘Over-exposure percents’ down to the minimum allowed, which is 0.1. For me that eliminated the patches of black.
Changing that also fixed it in digiKam, I suppose because they share a common codebase. What you see above are default settings.
If you’re wondering what this is even referring to, check Wikipedia.